The S. anginosus group (also known as the S. milleri group) is a subgroup of viridans streptococci that consists of three distinct streptococcal species: S. anginosus, S. intermedius, and S. constellatus; S. constellatus has two subspecies, S. constellatus subsp. constellatus and S. constellatus subsp. pharyngis [1-6]. These non-hemolytic viridans streptococci were first described by Guthof in 1956 after being isolated from dental abscesses. He named these organisms "Streptococcus milleri" in honor of the microbiologist W. D. Miller [1,7].

The organisms were subsequently recognized as normal flora of the human oral cavity and gastrointestinal tract with the ability to cause abscesses and systemic infections [8-12]. The unique characteristic of the S. anginosus group that sets these streptococci apart from other pathogenic streptococci, such as S. pyogenes (group A streptococcus) and S. agalactiae (group B streptococcus), is their ability to cause abscesses [13]. Unlike less virulent members of the viridans streptococci, members of the S. anginosus group should be considered true pathogens when isolated from humans, including children [14-16].

The microbiology, pathogenesis, clinical manifestations, and treatment of infections caused by members of the S. anginosus group in both adults and children will be reviewed here.

MICROBIOLOGY

Members of the S. anginosus group are gram-positive, catalase-negative cocci (like other members of the genus Streptococcus). They are non-motile, facultative anaerobes that demonstrate variable hemolysis patterns (alpha, beta, or gamma) on sheep blood agar [17]. Colonies are typically small (colony size less than 0.5 mm) [18]. Many strains demonstrate enhanced growth in the presence of CO2, whereas some strains may require anaerobic conditions.

Members of the genus Streptococcus may be classified by hemolysis patterns, Lancefield antigens, growth properties, and biochemical reactions. However, hemolysis patterns and Lancefield antigens are highly variable for members of the S. anginosus group. Almost 40 percent of S. constellatus and 12 percent of S. anginosus are beta-hemolytic, whereas 93 percent of S. intermedius strains are not beta-hemolytic [4]. Beta-hemolytic strains of S. constellatus typically react with Lancefield serologic group F antibody. Members of S. anginosus group often exhibit Lancefield antigens A, C, F, or G; strains containing the group F antigen may cross-react with other grouping sera. Group C and G streptococcus that belong to the S. anginosus group consist of small colony isolates; large colony group C and G streptococcus are discussed further separately. (See "Group C and group G streptococcal infection".)

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